Conventionally, as a type for electrically charging a surface of a photosensitive member in the image forming apparatus using the electrophotographic type, there is a contact charging type for subjecting the surface of the photosensitive member to a charging process by contacting a charging member, to which a voltage is applied, with the surface of the photosensitive member. For example, a charging device of the contact charging type using a charging roller which is a roller-shaped charging member has advantages such that a low voltage of a voltage source is realized and that an ozone generation amount is small.
As the contact charging type, further the following two types are well known. A first type is an “AC charging type” in which the photosensitive member is charged by applying a superposed voltage between a DC voltage and an AC voltage to the charging member. A second type is a “DC charging type” in which the photosensitive member is charged by applying only the DC voltage to the charging member.
In the “AC charging type”, the AC voltage uniformizes charging non-uniformity, so that a surface potential of the photosensitive member can be converged to a predetermined potential. For that reason, the “AC charging type”, can more uniformly charge the surface of the photosensitive member compared with the “DC charging type”. On the other hand, the “AC charging type” increases in electric discharge amount to the photosensitive member compared with the “DC charging type”, and therefore the surface of the photosensitive member is liable to abrade (wear). For that reason, when the photosensitive member is charged by the “AC charging type”, compared with the case where the photosensitive member is charged by the “DC charging type”, a lifetime of the photosensitive member becomes short in some cases. Further, in the “AC charging type”, an AC voltage source is needed. For that reason, it has been known that the “DC charging type” is advantageous in terms of a running cost and an initial cost compared with the “AC charging type”.
Further, conventionally, a pre-exposure means for removing residual electric charges on the surface of the photosensitive member after a toner image is transferred has been provided upstream of a charging portion of the charging device with respect to a surface movement direction of the photosensitive member. As the pre-exposure means, an LED chip array, a fuse lamp, a halogen lamp, a fluorescent lamp or the like is used. However, in the case where the above-described pre-exposure means is employed in an image forming apparatus employing the DC charging type in which the charging of the photosensitive member is effected by causing electric discharge under application of only the DC voltage to the charging roller as the charging portion, when for example a halftone image is formed, there if a problem such that a stripe-shaped density non-uniformity image (hereinafter also referred to as a “charging lateral stripe”) generates with respect to a longitudinal direction (direction substantially perpendicular to a circumferential direction) of the photosensitive member due to non-uniformity in surface potential of the photosensitive member. FIG. 17 is a schematic view for illustrating a mechanism of generation of the charging lateral stripe. A photosensitive drum 1 which is a rotatable drum-type (cylindrical) photosensitive member and a charging roller 2 which is a roller-shaped charging member are disposed in contact with each other. The photosensitive drum 1 and the charging roller 2 rotates so that movement directions of the respective surfaces move in the same direction at a contact portion (charging nip) a. Of minute gaps between the photosensitive drum 1 and the charging roller 2, a gap on an upstream side with respect to the surface movement direction of the photosensitive drum 1 is an upstream gap C1, and a gap on a downstream side is a downstream gap C2. At this time, in the upstream gap C1, a potential difference between the photosensitive drum 1 and the charging roller 2 exceeds a discharge start threshold based on the Paschen's law and the electric discharge is made, so that electric charges are placed on the photosensitive drum 1 and the surface potential becomes a dark portion (potential (VD). If the electric discharge is uniformly made in this upstream gap C1, as shown in (a) of FIG. 17, uniform charging of the photosensitive drum 1 is completed in the upstream gap C1 is completed, so that an image defect such as the charging lateral stripe does not generate. However, in the case of a constitution in which discharging exposure by the pre-exposure is made, even when the electric discharge is made between the photosensitive drum 1 and the charging roller 2 in the upstream gap C1 and uniform charging is completed, a potential of the photosensitive member charged on the upstream side lowers by the influence of dark decay when the photosensitive drum 1 passes through the nip between the photosensitive drum 1 and the charging roller 2 where the electric discharge is not made, so that incomplete (non-uniform) minute electric discharge is made between the photosensitive drum 1 reached the downstream gap C2 and the downstream gap C2, and at that portion, the surface potential of the photosensitive drum 1 generates non-uniformity and thus the charging lateral stripe generates. Therefore, in order to suppress generation of the above-described charging lateral stripe due to an increase in dark decay of the photosensitive drum 1 resulting from the pre-exposure means, it is also required that image formation is effected without providing this pre-exposure means. In the image forming apparatus in which the pre-exposure means is not provided, there is no means for actively discharging the surface of the photosensitive member after a charging process is ended, so that a density fluctuation generates in some cases. A detailed mechanism is not clarified, but sensitivity of the photosensitive member to light is improved by continuation of a state in which the photosensitive member is charged without effecting the electric discharge and a light portion potential (VL) after the exposure becomes low by the exposure of the photosensitive member to light during image formation, with the result that it is assumed that an image density increases. This phenomenon is also called VL down.
On the other hand, a method in which during a post-rotation operation after an image forming process is ended, a surface of a photosensitive member is exposed to light through one full circumference or more of the photosensitive member to effect electric discharge using an exposure device which is an exposure means (image exposure means) for forming an image by exposure the photosensitive member to light has been known (Japanese Laid-Open Patent Application Hei 4-93863).